n the past decades, the Internet has been booming and connecting people around the world. Current technology trends tell us that not only the people but also machine-type devices, e.g., household appliances, street facilities such as traffic lights, and connected vehicles, are going to be connected and communicate with each other. The Internet of Things (IoT) is a buzzword crossing all vertical industries. We are now, so to speak, in the dawn of the IoT era. With IoT technology, the world is getting increasingly smart. New concepts such as smart cities, smart homes, and smart agriculture are becoming a part of daily life and gradually changing our lifestyles.
Cellular IoT (CIoT) is considered one of the most attractive contributions to the IoT industry. The technologies referred to as licensed spectrum-based low-power wide-area (LPWA) access technologies are deployed in the GSM, LTE, or 5G new radio (NR) network and provide benefits with respect to quality of service, reliability, latency, and coverage range. Yet they also have the characteristics of low complexity, low cost, and low power consumption. CIoT technology provides the opportunity for enterprises to increase efficiency and improve value for the customer.
Standardized by the 3rd Generation Partnership Project (3GPP), CIoT is the general term for radio technologies known as LTE-M (for long-term evolution for machines) and NB-IoT (for narrow band-internet of things). As shown in Figure 2, the first machine type communication (MTC) standard based on LTE network (also called LTE-M) was specified in 3GPP Release 12. Starting with Release 13, 3GPP included NB-IoT technology under the scope of the standard. By the first NR Release 15, CIoT was already an integral part of the whole NR standardization effort.
There are two types of DRX deployments, namely, idle DRX (i-DRX) and connected DRX (c-DRX) that correspond to the UE’s radio resource control (RRC) idle and connected mode, respectively.
The UE operated in i-DRX mode monitors the PDCCH at defined time intervals. The UE will enter sleep mode between two consecutive PDCCH monitoring (see Figure 3).
In the c-DRX mode, the UE is allowed to monitor the PDCCH discontinuously to check if the scheduling messages can be detected by its cell radio network temporary identifier (C-RNTI) on PDCCH. Figure 4 illustrates the concept of a c-DRX process. Short DRX and long DRX cycles can be included in the c-DRX mode. The UE monitors the PDCCH during the On time and sleeps during the Off time in each DRX cycle. The DRX cycle starts when the DRX inactivity timer expires. The UE enters into a short DRX cycle(s) first, followed by a long DRX cycle. The adoption of a short DRX cycle is optional for LTE-M UE. However, NB-IoT UEs support only a long DRX cycle.
Figure 5 shows the basics of eDRX in comparison to the legacy DRX where the DRX cycle is extended from 2.56 seconds to minutes or even hours. In an RRC idle state, a UE can be configured for up to approximately 44 minutes for LTE-M and approximately 3 hours for NB-IoT.
To update the network about its availability, the UE performs periodic tracking area updates (TAU) after a configurable TAU timer (T3412 timer) has expired. After that, the UE stays reachable for paging in the idle state for a period of time (T3324 timer). Once the T3324 expires, the UE enters the deep sleep mode, also called power-saving mode (PSM), becomes dormant, and is therefore unreachable until the next periodic TAU occurs.
During the PSM, the UE turns off its circuitry but is still registered in the network, meaning that the UE still keeps the non-access stratum (NAS) status while closing the access stratum (AS) connection. The advantage of such an approach is that the UE can wake up immediately from the PSM without having to reattach or re-establish the packet data network (PDN) connections. This avoids extra power consumption due to the transmission of additional signaling messages required for establishing a higher layer connection.
Figure 6 on page 14 indicates the principle of the PSM and its message flow. The UE can exit PSM either after the expiration of the T3412 timer, i.e., renewed TAU, or when the UE initiates a mobile originated (MO) service or detach. With the latter, the UE can proactively exit PSM and enter the RRC idle state and connected state later to ask for the service.
The utilization of PSM is particularly interesting for use cases requiring infrequent mobile-terminated or mobile-originated events which allows the certain latency of the services, for example, a water meter sends the counter once a month. With a PSM mechanism, a 10-year battery lifetime, as recommended for the LTE-M and NB-IoT UEs, becomes possible.
Uplink and downlink data can already be sent together with message 3 (Msg3) and message 4 (Msg4), either piggybacked (CP-EDT) or multiplexed (UP-EDT) with the RRC message. The procedure could actually terminate after Msg4 if no more data has to be received or transmitted by the UE. The approach reduces the signaling overhead as well as message latency and therefore lowers the UE’s power consumption. Specifically, battery life can be improved by almost three years and the message latency is reduced by more than three seconds under poor radio conditions compared to performance under Release 13.
Certain maximum transport block size (TBS) is expected by EDT. For an LTE-M UE, TBS given in Msg3 is dependent on the coverage enhancement (CE) level. For CE0 and CE1, the UE can utilize the maximum 1000 bits TBS to transmit data, whereas for CE2 and CE3, the UE is only allowed to apply maximum 456 bits TBS. For an NB-IoT UE, the maximum TBS is about 1000 bits.
RRC message in Msg4 implicitly communicates whether more data has to be exchanged. By receiving the message “RRCEarlyDataComplete/RRCConnectionRelease,” the UE understands that the eNB has no more data to transmit and can go to RRC idle mode. Otherwise, by receiving the message “RRCConnectionSetup/RRCConnectionResume,” the UE will expect more data from eNB and fall back to legacy mode (Release 13) by establishing/resuming the connection.
Figure 7 shows the signaling flow of the CP-EDT and UP-EDT. The message flow plotted in the dotted line indicates the Release 13 CIoT UP and CP which serves the fallback procedure of the data transmission. This happens when more UE data is expected to be sent.
The latest development of WUS is addressed in 3GPP Release 16. A so-called UE-group WUS (GWUS) was introduced [5]. With this evolution, eNB instructs the UEs in the defined group to monitor the paging on PDCCH. The intention of this is to reduce the false alarm rate. In principle, several UEs may be mapped to the same PO. With the Release 15 WUS, some UEs may be unnecessarily awakened by the WUS when the intention of the eNB was actually to page the other UEs associated to the same PO.
After the UE is awake from a PSM or a DRX sleep mode, it usually needs to resynchronize with the network to acquire the time and frequency synchronization. This is typically due to the clock drift in the UE. In order to enable the UE to carry out fast time and frequency synchronization to save power, a newly designed resynchronization signal (RSS) is introduced. It is denser in time and frequency than the legacy PSS/SSS (still required for initial synchronization to a new cell). The potential power saving by using RSS can reach up to 98% in comparison to legacy PSS/SSS. Furthermore, RSS is also capable of indicating whether or not there are any changes in the MIB. Based on this, the UE may even skip decoding the MIB.
Furthermore, the UE may re-acquire SIB1 less often. This can be achieved by setting a flag bit in MIB indicating the change of SIB1 during the system information validity time. The UE shall read MIB after DRX or after cell reselection. If there is no indication of a change in SIB1, then the previous SIB1 stored in UE should be considered.
Improving the MIB and SIB demodulation performance is also addressed. The reduced acquisition time is enabled by enhanced cell global identity (CGI) reading delay requirements based on an accumulation of transmissions within two 40 ms periods for MIB and one modification period for SIB1/SIB2.
For NB-IoT UE operated in FDD mode, reduced system acquisition time is achieved during the cell access. This happens when eNB transmits SIB1-NB message (maximum 16 repetitions of SIB1-NB) in additional subframes on anchor carriers and non-anchor carriers. This approach enables the UE to decode the SIB1-NB faster, thus contributing to the power saving.
In this article, we touched upon a few of the common power-saving measures specified by 3GPP. They cover the feature optimization on the physical layer right up to the RRC layer. Different aspects such as reducing downlink monitoring (enter sleep mode to save power), faster release of the RRC connection, minimizing signaling overhead, and enabling the fast system acquisition can contribute to the overall power saving of the CIoT UE.
The evolution of the power saving techniques is an ongoing process and further enhancements are foreseen in the future IoT application based on non-terrestrial networks. Stay tuned and be excited.